Radiant wall heating systems are commonly employed in chemical, petroleum, and other industrial processes. A typical prior art radiant wall heating system 2 is illustrated in FIGS. 1 and 2. The prior art radiant wall system 2 comprises: a furnace, boiler, or other fired heater 4 having a housing 5, an outer wall 6, and a floor 8; a plurality of process tubes 10 which carry the process fluid through the housing 5; an upwardly extending radiating wall 12 within housing 5; and at least one radiant wall burner 14. The radiant wall 12 within heater 5 is typically comprised of a radiating ceramic tile material or other material which will radiate the combustion energy generated by burner 14 toward and onto the process tubes 10.
The prior art burner 14 shown in FIGS. 1 and 2 comprises: a burner housing 16 positioned primarily outside of the heater wall 6; a burner wall 18 which extends horizontally from burner housing 16 through the heater wall 6 and into the interior of the furnace housing 5; a combustion air flow passage 20 extending through burner housing 16 and burner wall 18; a damper or other regulating device 22 in burner housing 16 for regulating the flow of combustion air 26 through the burner 14; an upwardly facing flow passage opening 24 provided through the upper end of the burner wall 18 for delivering the combustion air 26 upwardly as illustrated in FIG. 1; a plurality of primary fuel ejectors 28 for ejecting some (typically most) of the burner fuel into a primary combustion stage 30; and a plurality of secondary fuel ejectors 32 for ejecting the remainder of the burner fuel into a secondary combustion stage 34. The combustion air 26 will typically be delivered to the burner 14 by forced circulation, natural draft, or a combination thereof. Although the prior art burner assembly 14 shown in FIG. 1 extends horizontally through the heater wall 8, it is also known in the art to extend the burner assembly vertically through the floor 8 of the heater.
Each of the fuel ejectors 28 and 32 will typically comprise a fuel ejection tip 36 or 38 secured on a vertical end portion of a fuel pipe 40 or 42. Each ejector tip 36 and 38 has one or more orifices or other flow ports provided therein for ejecting fuel in a desired direction and pattern. The ejection tips 38 provided on the secondary fuel ejectors 32 will typically be effective for ejecting fuel upwardly into a flat flame combustion stage 34 against the radiating wall 12.
As shown in FIG. 2, the upper end of the burner wall 18 provides a periphery 44 which surrounds and establishes the boundaries of the upwardly facing combustion air opening 24. A first (near) side 46 of the periphery 44 is positioned closest to the radiating wall 12 and establishes a near boundary 48 of the combustion air opening 24. A second (outer) side 50 of the periphery 44 is positioned furthest from the radiating wall 12 and establishes an outer boundary 52 of the combustion air opening 24. The near boundary 48 of opening 24 will include or consist of one or more “closest” point(s) 49 which is/are closer than any other portion of the upper opening 24 to the radiating wall 12. In like manner, the outer boundary 52 includes or consists of one or more “furthest” point(s) 53 which is/are further than any other portion of the upper opening 24 from the radiating wall 12.
As shown in FIG. 2, the upper discharge end of burner 14 has a rectangular shape so that the near boundary 48 of the upper opening 24 is a straight line segment which is adjacent to and runs parallel to the radiating wall 12. Because all portions of the near boundary line 48 are equidistant from the radiating wall 12, each point on line 48 is therefore a near boundary point “closest” to the radiating wall 12. Similarly, the outer boundary 52 of the upper opening 24 is also a straight line segment running parallel to the radiating wall 12. Thus, each point on line 52 is an outer boundary point which is “furthest” from the radiating wall 12. The linear outer boundary 52 and near boundary 48 of combustion air opening 24 are spaced apart a maximum width 54, as shown in FIG. 2, perpendicular to the radiating wall 12.
As illustrated in FIGS. 1 and 2, at least some of the ejectors 28 and 32 employed in the radiant wall burners heretofore known in the art are commonly positioned either in or beyond the outer peripheral wall 50 of the combustion air opening 24. Thus, the ejectors 28 will typically be spaced outwardly from the near boundary line 48 of the upper opening 24 by a distance 58 which exceeds the maximum width 54 of the opening 24.
As indicated above, the prior art radiant wall burner 14 is a staged fuel burner having a primary stage combustion zone 30 and a secondary stage combustion zone 34. An intended objective of the staged fuel burner is to lower the amount of NOX emissions produced in the combustion process. In the staged fuel design, excess air is typically present in the primary combustion stage 28 so that the overall temperature of the burner flame is lowered and the production of NOX compounds is thereby reduced.
Unfortunately, in the radiant wall burners heretofore used in the art, flue gas currents 60 within the heater 4 commonly act to pull the combustion flame 30 produced by ejectors 28 outwardly away from the radiating wall 12. This reduces the efficiency, effectiveness, and stability of the burner 14 and also reduces the overall efficiency and heating capacity of the radiant wall system 2. In addition, it is not uncommon that the flue gas currents 60 will pull the flame 30 outward to such a degree that it is very close to and/or impinges upon the process tubes 10. The impingement or near impingement of the burner flame 30 on the process tubes further diminishes the performance and reduces the efficiency of the heating system, can damage the process tubes 10 or other internal components, and can result in accelerated coke production and lay down within the tubes 10.
Thus, a need exists for an improved radiant wall burner and a better method for operating radiant wall systems which will provide greater flame stability and will prevent or at least significantly reduce the flame drift and impingement problems experienced with the prior art burners. The improved radiant wall burner and method will preferably also be effective for maintaining low NOX production rates and will most preferably be effective for further reducing NOX emissions.